1. Field of the Invention
The present invention relates to dielectric filters for use in mobile communication systems for the microwave band, and duplexers having such filters incorporated therein for use in radio devices.
2. Description of the Related
Conventional dielectric filters wherein coaxial resonators are used include, for example, the one disclosed in Examined Japanese Utility Model Publication No. 44566/1987 (FIG. 28). The disclosed dielectric filter comprises a plurality of quarter-wavelength coaxial resonators 1 each including a dielectric member 12 which has a through hole 4. The outer peripheral surface of the dielectric member and the inner peripheral surface thereof defining the through hole are covered with an electrically conductive material to provide an outer conductor 5 and an inner conductor 6, respectively. The dielectric member has an open end face 1a where the outer conductor 5 and the inner conductor 6 each have an open end, and a short-circuit end face 1b where the other ends of the conductors are short-circuited.
A connecting member comprising a dielectric bush 14 and a connecting bar 15 is fitted in the through hole 4 of each coaxial resonator 1, and the other end of the bar 15 is joined to a coupling electrode 13 on a substrate 16, whereby the dielectric filter is capacitance-coupled to an external circuit.
It has been required in recent years that mobile communication devices be made smaller in size and lightweight. To comply with this requirement, the dielectric filter, one of the components of these devices, also need to be made more compact.
In providing compact dielectric filters, the ratio of the diameter of the inner conductor 6 to the outer conductor 5 must be 3.6 in order to obtain a high Qu value (no-loaded Q factor). If the diameter of the outer conductor 5 is up to 4 mm, the diameter of the inner conductor 6 is up to 1.1 mm, whereas extreme difficulties are encountered in the prior art in inserting the connecting members 14, 15 into the through hole 4 of the coaxial resonator for connection to the external circuit 13. Thus, making the dielectric filter compact becomes difficult.
In mobile communication devices, on the other hand, signals of different frequencies are separated according to the frequency or are combined together using duplexers. Such duplexers comprise a transmitting dielectric filter and a receiving dielectric filter which are different in center frequency. With the trend of mobile communication toward higher frequencies, the difference between the receiving band and the transmitting band in the center frequency becomes smaller, making it difficult for these dielectric filters to attain the desired attenuation outside the pass band. Accordingly, the characteristics of the dielectric filters for use in the duplexer must involve a local minimum of attenuation.
The present applicant filed a patent application with the Patent Office of Japan for a dielectric filter which is free of the above problem and which has the construction shown in FIG. 29 (Japanese Patent Application No. 46796/1991). A U.S. patent has been granted for the filter as U.S. Pat. No. 5,144,269.
This dielectric filter comprises a plurality of coaxial resonators 1 arranged side by side and each having a dielectric member 12 formed with a through hole 4. The outer and inner peripheral surfaces of the dielectric member 12 are covered with a conductive material to provide an outer conductor 5 and an inner conductor 6, respectively. The resonator has a short-circuit end face and an open end face, in the vicinity of which the outer conductor 6 is partially removed, along with a portion of the dielectric member when so desired, to form a recess 17. A dielectric substrate 19 provided with an external connection electrode 18 is attached to the recessed portion 17.
At least three coaxial resonators are used in the filter to provide a local minimum of attenuation. More specifically, a capacitance is formed between the external connection electrode 18 and a capacitance-forming electrode provided on the dielectric substrate 19 to obtain frequency characteristics involving a local minimum in the attenuation region.
With the filter described above, however, the resonator needs machining for forming the recessed portion 17 for attaching the dielectric substrate thereto and therefore can not be made compact without limitations. The characteristics of the filter also have the problem that sufficient suppression is not available outside the pass band.
The dielectric filter has another problem in that the coaxial resonator can not be reduced in its overall length because the substrate 19 needs to be partly projected from the dielectric member 12 for coupling to an external circuit.